I know it can be difficult to check serial numbers at a show, but I would not risk the penalties for getting a raw deal on one that was not assembled at the factory without documentation.
It is an easy task for the feds to check as it is for you! Case in point, I recently came across a "STEAL OF A DEAL" at a local pawn shop($700.00)!!!!! For a Pre ban Eagle M4c.

I THREW!! my money down as fast as i could get it out of my pocket and preceeded to email Eagle to check if it had left the factory as a rifle or receiver?" You know I was crossing my fingers!!" sure nuff! It was go!! If it had'nt I dont think I would of picked it up.
Sorry bout the off topic but im still jazzed and had to tell ya all